In the metallurgical industries it is a common practice in the production of alloys to add a metal to be alloyed directly to a molten bath of the metal with which it is to be alloyed wherein the metal to be alloyed dissolves in the molten bath thereby forming the desired alloy in molten form. In many instances, the metal to be added to the bath has a vapor pressure at the temperature of the molten bath to which it is added which exceeds the total ambient pressure and is readily oxidizable. In these instances, the addition of the metal to be alloyed to the molten metal bath results in vaporization and oxidation of the metal added resulting in losses of the metal addition and hence reduced recoveries of the metal addition in the bath as well as a safety hazard caused by the accompanying flare and fumes. The economic impact of such metal losses in commercial scale operations is significant and the reduction of such vaporization losses by even one percent based on the weight of the metal or alloy thereof added to the molten bath can result in considerable cost savings in a plant scale operation.
A reactive metal will therefore be defined as a metal added in elemental or alloy form to a molten metal or metal alloy bath the temperature of which is such that the vapor pressure of the metal addition at the bath temperature exceeds the total ambient pressure with such addition resulting in the vaporization of the added metal. The term reactive metal includes reactive metal base alloys. Reactive metal additions referred to herein are those reactive metals which float on the bath and are gradually dissolved therein.
Examples of the addition of reactive metals to a molten metal bath which are common in industrial practice include: Magnesium (boiling point=1107.degree. C.) which is added to a molten ferrosilicon bath to form alloys containing about 1 to 9 percent magnesium by weight for the purpose of producing nodularizing alloys for cast iron. In this instance, the temperature of the molten ferrosilicon bath typically ranges from about 1320.degree. C. to 1600.degree. C. at which temperatures the vapor pressure of magnesium ranges from 3514 to 14800 mm Hg while the total ambient pressure would be approximately 760 mm Hg (i.e. approximately standard atmospheric pressure). Such additions result in vaporization of the added magnesium and hence an accompanying loss of magnesium metal.
Calcium (boiling point=1440.degree. C.) is added to molten iron or steel baths at temperature of approximately 1600.degree. C. for the purpose of desulfurization and deoxidation. At a temperature of 1600.degree. C., the vapor pressure of calcium is 2980 mm Hg thus exceeding the approximate total ambient pressure of 760 mm Hg which results in the vaporization of the calcium addition.
Magnesium is added to molten aluminum silicon baths at temperatures of approximately 1400.degree. C. for the purpose of making magnesium aluminum silicon casting alloys. At a temperature of 1400.degree. C., the vapor pressure of magnesium is 5570 mm Hg thus exceeding the approximate total ambient pressure of 760 mm Hg which results in vaporization of the magnesium addition.
Strontium or calcium is added to molten ferrosilicon at bath temperatures which would frequently exceed the boiling point of strontium (1380.degree. C.) or calcium (1440.degree. C.) for the purpose of making nodularizing or inoculant alloys for cast iron. In these instances, the vapor pressure of the strontium or calcium would exceed the total ambient pressure resulting in vaporization of the strontium or calcium addition.
Magnesium is added to molten iron in small quantities for the purpose of nodularizing the graphite in the iron. In this instance, the temperature of the molten iron bath typically ranges from about 1350.degree. C. to 1500.degree. C. thus exceeding the boiling point of magnesium (1107.degree. C.) and therefore the vapor pressure of the magnesium exceeds the total ambient pressure resulting in the vaporization of the magnesium addition.
It is known in the prior art that gaseous sulfur hexafluoride (SF.sub.6), an odorless, colorless, nontoxic gas, is effective in dilute quantities for providing a protective atmosphere for magnesium melting and casting operations. Such melting and casting operations are typically carried out at temperatures of about 500.degree. to 700.degree. C. with the temperature being largely dependent on the melting temperature of the magnesium or the magnesium alloy which is the subject of the operation. However, these temperatures are significantly below the boiling point of magnesium (1107.degree. C). Such uses of sulfur hexafluoride are described, for example, in U.S. Pat. No. 4,089,678--Hanawalt, U.S. Pat. No. 3,400,752--Unsworth and Proceedings of the International Magnesium Association, May 22-24, 1977, "Melting Magnesium Under Air/SF.sub.6 Protective Atmosphere," pages 16-20, S.L. Couling et.al. However, the known prior art references do not discuss the use of dilute SF.sub.6 atmospheres in conjunction with the handling of reactive metals at temperatures at which the vapor pressure of the reactive metal exceeds the total ambient pressure.
Prior art methods concerning the addition of reactive metals to molten metal baths include U.S. Pat. No. 3,545,960--McClellan, the disclosure of which is incorporated herein by reference, which discloses the agitation of a molten metal bath to which a solid material is added by injecting a nonreactive gas, for example nitrogen, below the surface of the bath to effect the homogeneous intermixing of the metal bath and the solid addition. Practice of such a method would establish a nonreactive gas atmosphere over the bath.